Is this IMO number valid?

Formula, assumptions, and limits

IMO 9074729 is valid: its first six digits, weighted 7 down to 2, sum to 139, and 139 ends in the 9 the number ends in. The same arithmetic shows that IMO 8712345, the format example printed in resolution A.1117(30) itself, is not a valid number: its sum is 128, which calls for a check digit of 8, not 5.

Formula

The number is valid when $C = d_7$.

• $d_1 \ldots d_6$: the first six digits, read left to right, each 0 to 9.
• $w_i$: the weight for position $i$, from 7 at the first digit to 2 at the sixth.
• $d_7$: the seventh digit as printed.
• $C$: the check digit the first six digits call for; the last digit of the weighted sum.

Derivation

The number series predates the IMO scheme. The S&P Global IMO Ship Numbering FAQ, published by the IMO, records that Lloyd’s Register introduced permanent ship numbers in 1963, made them seven digits in 1969, and that this series was adopted as the basis of the IMO number in 1987. The same FAQ describes each number as a six-digit sequential number followed by a check digit, by which the integrity of the number can be verified.

None of the three IMO resolutions that have set the scheme states the check. A.600(15) of 19 November 1987, A.1078(28) of 4 December 2013 and the current A.1117(30) of 6 December 2017 fix only the format: the letters IMO in front of seven digits, allocated at build or when a ship is first included in a register (annex para 3). The FAQ does not give the weights either. The weights 7 to 2 and the last digit of the sum are the convention that published validators apply, and this site checked it on 13 October 2026 against the 1,542 different vessel IMO numbers on the US Treasury OFAC sanctions list: every one passes.

Assumptions

• The input is a ship number under A.1117(30), not a company or registered owner number under MSC.160(78), which does not follow the ship check digit.
• The number lies between 1000000 and 9999999, the range the S&P Global FAQ gives, so a number starting with 0 is rejected.
• The letters IMO, spaces, dots and hyphens are formatting and are removed before the check.
• Exactly seven digits make a number; six digits are read as a number missing its check digit.

Worked example

Take IMO 9074729.

1. Weight the first six digits: 9×7 = 63, 0×6 = 0, 7×5 = 35, 4×4 = 16, 7×3 = 21, 2×2 = 4.
2. Add them: 63 + 0 + 35 + 16 + 21 + 4 = 139.
3. The last digit of 139 is 9, and the seventh digit is 9. Valid.

Swap the second and third digits to 9704729 and the sum becomes 146, so the check digit should be 6; the number fails. Every swap of two neighboring digits among the first six fails the same way: neighboring weights differ by one, so the sum moves by the difference between the two digits, which is never a multiple of 10. A swap of the sixth digit with the check digit fails too, because the sum then moves by three times the difference, and 3 shares no factor with 10.

Edge cases and limits

A single mistyped digit gets through one time in nine, taking every position and every wrong value as equally likely, because some weights share a factor with 10. Changing the third digit, weight 5, by 2, 4, 6 or 8 moves the sum by a multiple of 10: 9094729 passes the check as well as 9074729. A wrong second, fourth or sixth digit gets through only when it is off by exactly 5. The first and fifth digits, weights 7 and 3, and the check digit itself never let a single error through.

The check says nothing about whether the number has been issued. S&P Global had assigned 453,524 of the 900,000 numbers between 1000000 and 9999999 that carry a valid check digit by 30 September 2022, according to its FAQ, and one random seven-digit string in ten passes the check. A passing number can still be unassigned, or belong to a different ship than the one on the document.

Regulatory basis

SOLAS regulation XI-1/3 made the number mandatory from 1 January 1996 for passenger ships of 100 GT and upwards and cargo ships of 300 GT and upwards (IMO Circular Letter No.1886/Rev.7, 28 November 2022). Resolution A.1117(30), which revoked A.1078(28), sets the scheme and extends it on a voluntary basis to ships of 100 GT and above including fishing vessels; passenger ships under 100 GT, high-speed passenger craft and mobile offshore drilling units on international voyages; and motorized inboard fishing vessels down to 12 m length overall authorized to operate outside waters under the flag State’s jurisdiction. It excludes ships without mechanical propulsion, pleasure yachts, ships on special service, hopper barges, floating docks, ships of war and troop ships, and wooden ships other than fishing vessels (annex para 2).

Common errors

• Reading the digit O for zero, or I for one, when copying from a certificate or a hull marking.
• Checking a company number from an ISM Document of Compliance against the ship check: it fails, and it is not wrong.
• Taking the MMSI for the IMO number. The MMSI has nine digits and identifies a radio station, under ITU-R Recommendation M.585-9.
• Treating a passing check digit as proof of the ship’s identity.

Ship number or company number

Since 1 January 2009, ISM and ISPS certificates issued or renewed carry a second IMO number: the company and registered owner identification number. Resolution MSC.160(78) set up the scheme in 2004, and SOLAS regulation XI-1/3-1, added by resolution MSC.194(80), made it mandatory. It is printed with the same IMO prefix, so the two are easy to confuse on a Document of Compliance or a Continuous Synopsis Record. Company numbers do not follow the ship check digit: of 34 on the OFAC list, 4 pass it, about the rate chance alone gives. If a number from a company document fails here, confirm which kind it is before treating it as a typing error.

Where the number is marked on the ship

The 2002 SOLAS Conference amended regulation XI-1/3 to require the number to be marked permanently on the ship, in force from 1 July 2004 (SOLAS/CONF.5/32, resolution 1). Ships built before that date had to comply by their first scheduled dry-docking after it.

The marking is made by raised lettering, by cutting in or center punching, or by an equivalent method that keeps it from being easily removed, in a contrasting color clear of other hull markings (para 5). Where the hull marking and a certificate disagree by one digit, the check digit usually shows which of the two was copied wrong.

Where to find each input

Resolution A.1117(30) asks for the number in the box headed “Distinctive number or letters” on the ship’s certificates, beside the call sign (para 7), and the hull carries it in the two markings above. Commercial documents, from the charter party to the bill of lading, usually quote it beside the ship’s name. The IMO’s GISIS Ship and Company Particulars module confirms which ship a number was assigned to; it needs a free public account.